Single Institution Preliminary Evaluation of a National Study for the Development of Daily Online Magnetic Resonance Image Guided Radiotherapy

PURPOSE/OBJECTIVE(S)

A 4-stage non-comparative prospective feasibility study to assess and develop imaging protocols for MRIgRT was opened at the first two centers using MR Linac technology in the UK. The primary aims of this study were to determine a) the acceptability of MR images for target and organ at risk delineation and registration; b) inter/intra observer registration and delineation variation. This work reports on the initial results from a single center.

MATERIALS/METHODS

In June 2019, following ethical and regulatory approvals the 2nd UK centre began study recruitment as follows: Stage A: non-patient volunteer imaging to determine sequence suitability for normal tissue in 6 anatomical sites (head & neck (H&N), chest wall/breast, lung/esophagus, abdomen, male and female pelvis). Volunteers were recruited in cohorts of 3 participants per region, and image quality was assessed by 3 independent observers using a visual guidance assessment tool (VGA). Stage B: the most suitable sequences defined in stage A used to assess the visibility of targets/normal tissues in patient volunteers using the same methods as in stage A. Stage C: patient volunteers were imaged using sequences selected from stage B to determine inter and intra observer segmentation and registration variation. Stage D recruitment of patient and non-patient volunteers for further image develop and refinement of MRIgRT workflows. All participants completed experience questionnaires to optimize workflows. Participants were asked to undergo 1-12 imaging sessions, lasting no more than 60.

RESULTS

To date 151 participants (61 non-patients; 90 patients) have undergone 231 imaging sessions. From stage B, vendor provided, in-workflow sequences have been agreed from 47 completed VGAs by prioritizing high scores in either the tumor (e.g., lung) or organs at risk (e.g., cervix). T2w 3D sequences scored best in cervix, pancreas, prostate, bladder, liver, soft-tissue metastases and rectal cancers; T1w 3D sequences for H&N, and patient a specific approach for lung. No suitable sequences have been selected for partial breast. Research sequences (e.g., diffusion weighted or motion corrected imaging) have been agreed or are in development in stages C & D for H&N, cervix, bladder and prostate cancers. The mean interobserver (n = 8) vector variation in 5 H&N patients was largest (3.6mm) using T1-CT boney registrations and smallest (2.1mm) using T1-T1 soft-tissue registrations (mean observer match confidence 3.7/5). Analyses using MR to CT, MR to MR and CT to CT registrations in lung, pancreas, cervix, bladder, and prostate have also been completed. Interobserver delineation studies are on-going.

CONCLUSION

Using a 4-stage non-comparative prospective feasibility study has facilitated clinical implementation MRIgRT of multiple treatment sites at our institution.

The Impact of Intra-thoracic Anatomical Changes upon the Delivery of Lung Stereotactic Ablative Radiotherapy

AIMS

So far, the impact of intra-thoracic anatomical changes (ITACs) on patients treated with stereotactic ablative radiotherapy (SABR) for early-stage non-small cell lung cancer is unknown. Studying these is important, as ITACs have the potential to impact the workflow and reduce treatment quality. The aim of this study was to assess and categorise ITACs, as detected on cone beam computed tomography scans (CBCT), and their subsequent impact upon treatment in lung cancer patients treated with SABR.

MATERIALS AND METHODS

CBCTs from 100 patients treated with SABR for early non-small cell lung cancer were retrospectively reviewed. The presence of the following ITACs was assessed: atelectasis, infiltrative change, pleural effusion, baseline shift and gross tumour volume (GTV) increase and decrease. ITACs were graded using a traffic light protocol. This was adapted from a tool previously developed to assesses potential target undercoverage or organ at risk overdose. The frequency of physics or clinician review was noted. A linear mixed effects model was used to assess the relationship between ITAC grade and set-up time (time from first CBCT to beam delivery).

RESULTS

ITACs were observed in 22% of patients. Twenty-one per cent of these were categorised as 'red', implying a risk of underdosage to the GTV. Most were 'yellow' (51%), indicating little impact upon planning target volume coverage of the GTV. Physics or clinician review was required in 10% of all treatment fractions overall. Three patients needed their treatment replanned. The mixed effect model analysis showed that ITACs cause a significant prolongation of set-up time (Χ2(3) = 9.22, P = 0.02).

CONCLUSION

Most ITACs were minor, but associated with unplanned physics or clinician review, representing a potentially significant resource burden. ITACs also had a significant impact upon set-up time, with consequences for the wider workflow and intra-fraction motion. Detailed guidance on the management of ITACs is needed to provide support for therapeutic radiographers delivering lung SABR.